THE TRANSPORTATION OF LIQUIDS 141
Keep the stuffing boxes well packed with a good quality of packing. Screw up the gland no tighter than necessary to stop leakage, and renew the packing before it becomes hard. Watch the rods and the outside-packed plungers to detect cutting, If pump is provided with a surface condenser, never exhaust into an empty condenser, as this will destroy the tube packing.
Operating Centrifugal Pumps.—Centrifugal pumps are primed before starting, either by means of an ejector with the discharge closed, or if a foot valve is provided at the end of the suction pipe, it may be filled with water through a priming pipe, the air escaping through a vent at the highest point of the casing. Start and bring up to speed, then open discharge valve. Always close the discharge valve before starting and stopping.
The stuffing boxes should be packed with square graphite packing. Round packing will wear grooves in the shaft. Obtain from the manufacturer a set of characteristic curves and compare with actual performance. If the readings do not agree with the curves examine pump for defects, such as stones in the impeller or broken blades. These curves will also assist in running the pump near the point of maximum efficiency.
Setting up Pumps.—The suction pipe should be large and as short and direct as possible. Bends should be of long radius. Lay to a uniform grade, rising towards the pump, so as to avoid air pockets. In connecting the steam pipe make due allowance for expansion. Place a throttle valve in the pipe close to the pump and provide means for draining the pipe with the valve closed. Before connecting the pipe to the steam chest blow out the steam thoroughly to keep dirt from getting into the cylinder.
The nozzles of a centrifugal pump as furnished by the manufacturer are necessarily computed for fairly high velocities. Increasers should be attached and the pipes should be made large enough to give velocities of from 4 to 5 ft. per second in the discharge pipe and from 3 to 4 ft. per second in the suction pipe. A strainer of liberal proportions should be at the bottom of the suction pipe, and the free opening through the strainer should be from 2 to 4 times that of the pipe. The openings through the strainer should be smaller than the smallest passage in the impeller. Install a gate valve and a check valve near the discharge nozzle with suitable drains, priming pipes and a pressure gage.